The warning silenced was:
libavfilter/af_join.c: In function ‘join_request_frame’:
libavfilter/af_join.c:451:9: warning: ‘nb_samples’ may be used uninitialized in this function [-Wuninitialized]
It is not used in any filters currently and is inherently evil. If
passing binary data to filters is required in the future, it should be
done with some AVOptions-based system.